1. Field of the Invention
The present invention relates to a built-in drive power-source semiconductor device, applicable for a power switching device, in which a drive power source can be obtained from a load power source. More particularly, the invention relates to an arrangement of a charging circuit fabricated into a semiconductor device. The present invention relates to improvements in the charging circuit of a built-in drive-power-source semiconductor device having an output insulated gate semiconductor element, a gate drive circuit for charging and discharging the gate of the insulated gate semiconductor element, a capacitor for supplying a power source to the gate drive circuit, and a charging circuit for charging the capacitor.
1. Discussion of the Related Art
FIG. 6 shows the circuit arrangement of a prior art built-in drive-power-source semiconductor device 1, proposed in Japanese Patent Application No. 1-266852 by the Assignee of the Present Patent Application. In the semiconductor device, an output switch portion 10 employs an output MOSFET 2 as an output insulated gate semiconductor element. The drain electrode or the source electrode of the MOSFET 2 is connected to a load (not shown). An output control portion 20 for driving the gate of the output MOSFET 2 to control the output switch portion 10 includes an enhancement MOSFET 32 for charging the gate of the output MOSFET 2, and a depletion MOSFET 35 for discharging the gate of the output MOSFET 2.
Both the MOSFETs 32 and 35 are connected in series. The drain electrode of the enhancement MOSFET 32 is connected to a power source receiving terminal 21 of the output control portion 20. The source electrode of the depletion MOSFET 35 is connected to terminal Pl of the load terminals of the semiconductor device, which is connected to the source electrode of the output MOSFET 2. In the output control portion 20, a parallel circuit comprising a photo diode array 29 and a resistor 31 is connected between the gate and source electrodes of the enhancement MOSFET 32. A parallel circuit comprising a photo diode array 33 and a resistor 34 is connected between the gate and source electrodes of the depletion MOSFET 35. Photo diode arrays 29 and 33 are disposed so that generate photo voltage in response to light received from the LED 13.
A battery portion 30 for providing a power source for the output control portion 20 consists of a capacitor 3. The high potential electrode 3a of the capacitor 3 is connected to the power source receiving terminal 21. A charging portion 40 for charging the capacitor 3 of the battery portion 30 during the rest or nonconductive period of the output MOSFET 2 comprises a charging circuit connected between load terminal Ph coupled with the drain electrode of the output MOSFET 2 and another load terminal Pl coupled with the source electrode of the output MOSFET 2 during the rest or nonconductive period of the output MOSFET 2. The charging portion 40 constitutes a charging MOSFET 5 and a reverse current blocking diode 6, which are connected in series between the drain electrode of the output MOSFET 2 and the high potential electrode 3a of the capacitor 3, and a current restricting resistor 7 and a first constant voltage diode 41, which are connected in series between the drain and source electrodes of the output MOSFET 2. The anode of the first constant voltage diode 41 is connected to the source electrode of the output MOSFET 2, and the cathode of the diode is connected to both the resistor 7 and the gate of the charging MOSFET 5. Thus, the first constant voltage diode 41 functions to drive the gate of the charging MOSFET 5.
To operate the output MOSFET 2 of the built-in drive-power-source semiconductor device, the LED 13 is turned on. When the LED 13 is turned on, the photo diode arrays 29 and 33 generate photo voltages. The photo voltage generated by the photo diode array 29 renders the enhancement MOSFET 32 conductive. A negative voltage is applied between the gate and source of the depletion MOSFET 35, so that the MOSFET 35 is rendered nonconductive. Accordingly, the voltage across the capacitor 3 is applied to the gate of the output MOSFET 2, thereby charging the gate and in turn operating the output MOSFET 2. Consequently, the load terminals Ph and Pl coupled with the output MOSFET 2 are connected. In this way, the semiconductor device functions as a switch.
To turn off the output MOSFET 2, the LED 13 is turned off. When the LED 13 is turned off, the photo voltages from both the diode arrays 29 and 33 disappear, so that the enhancement MOSFET 32 is rendered nonconductive, and the depletion MOSFET 35 is rendered conductive. As a result, the application of the voltage across the capacitor 3 to the gate of the output MOSFET 2 is stopped. The charge stored in the gate is discharged through the depletion MOSFET 35, so that the output MOSFET 2 becomes nonconductive.
The operation of the charging portion 40 will now be described. It is assumed that a high voltage substantially equal to the voltage of a load power source is applied across the source-drain path of the output MOSFET 2, and that no charge is stored in the capacitor 3. In this case, the potential of the source electrode is zero, and the potential at the gate of the MOSFET 5 takes a value determined by a voltage value across the first constant voltage diode 41. Since the voltage between the gate and source of the charging MOSFET 5 has been set to be in excess of the gate threshold value of the MOSFET 5, the charging MOSFET 5 becomes conductive. As a result, the capacitor 3 is charged by the voltage between the drain and source of the output MOSFET 2 through the charging MOSFET 5 and the reverse current blocking diode 6, so that the high potential electrode 3a of the capacitor is charged to a high potential. When the charging operation progresses and the potential at the high potential electrode 3a of the capacitor 3 is increased, the voltage between the gate and source of the charging MOSFET 5 decreases. As a result, the charging MOSFET 5 becomes nonconductive and the charging of the capacitor 3 is stopped. The potential increase at the high potential electrode 3a of the capacitor 3 stops. Accordingly, the potential at the high potential electrode 3a of the capacitor 3 is kept at a fixed value substantially equal to the difference between the voltage value across the first constant voltage diode 41 and the gate threshold value of the charging MOSFET 5. The voltage across the capacitor 3, which is kept at the fixed value, is applied to the power source receiving terminal 21 of the output control portion 20, and is used as a drive power source. The charge stored in the capacitor 3 is discharged through the charge/discharge at the gate of the output MOSFET 2, so that the potential at the high potential electrode 3a of the capacitor 3 decreases. However, the capacitor is charged again in the next rest period of the output MOSFET 2, and is kept at the fixed value.
In the above prior art built-in drive power-source semiconductor device, the power-supplying capacitor for controlling the output MOSFET can be charged from an external circuit connected as a load to the output MOSFET. Therefore, there is no need for a special external power supply. The semiconductor device can be used as a semiconductor device having an on-board drive power source. In this respect, the built-in drive power-source semiconductor device is suitable for use as a switching element in a power source circuit provided independently of the control circuit portion.
A necessary improvement in this semiconductor device is the switching characteristic. In the conventional built-in drive-power-source semiconductor device, when the charging operation across the capacitor 3 progresses and the potential at the high potential electrode 3a increases, the voltage between the gate and the source electrodes of the charging MOSFET 5 decreases. The MOSFET 5 has an output having a saturation characteristic as exemplarily shown in FIG. 7. As shown, it exhibits a constant current characteristic in the region where the drain-source voltage V.sub.DS is high. As also shown in FIG. 7, the current value decreases as the gate-source voltage V.sub.GS decreases, for example, from V.sub.GS6 to V.sub.GS1. Thus, in the conventional built-in drive-power-source semiconductor device, as the charging of the capacitor 3 progresses and the gate-source voltage V.sub.GS decreases, the charging ability of the charging MOSFET 5 is reduced.
As an example, it is desirable that variations in the power source voltage supplied to the output switch portion 10 to drive the output control portion 20 be suppressed in order to improve the switching characteristic of the semiconductor device. Accordingly, variations in the potential at the high potential electrode 3a must be suppressed. However, when the capacitance of the capacitor 3 is set to a large value in order to suppress variations in the power source voltage, the charging time is drastically increased because the voltage V.sub.GS between the gate and source electrodes of the charging MOSFET 5 is always low. As a result, it is impossible to increase the drain current to charge the capacitor 3 through the charging MOSFET 5, and the charging time is increased. For this reason, when the on-state time of the output MOSFET 2 is long and the off-state time is short, the capacitor 3 is insufficiently charged. Accordingly, the voltage to drive the output control portion 20 drops, resulting in insufficient charge of the gate of the output MOSFET 2. Due to the increased loss generated in the output MOSFET 2, it is impossible to obtain the switching characteristic required by the control signal.
In a case in which the charging MOSFET has a satisfactory charging capability, the capacitor can be sufficiently charged within the period that the drain-source voltage of the output MOSFET 2 remains low immediately after the MOSFET 2 is turned off. Accordingly, the loss generated in the charging MOSFET 5 can be reduced.
On the other hand, in a case in which the charging MOSFET has an unsatisfactory charging capability, the capacitor 3 continues to be charged even after the drain-source voltage of the output MOSFET 2 has reached a high value after the MOSFET 2 is turned off. This leads to an increase in the loss generated in the charging MOSFET 5.
To solve the above problems, it has been proposed that a high-voltage diode be used as the constant voltage diode 41 so that the charging MOSFET 5 charges the capacitor when its gate-source voltage V.sub.GS is high, in order to improve the switching characteristic. As a result, the capacitor 3 is rapidly charged and the potential at electrode 3a becomes high. Therefore, the increased losses generated in the charging MOSFET can be eliminated. However, this solution results in an increased cost of the overall device. In the above configuration, high potentials are applied between the gate and source of the output MOSFET 2, between the corresponding electrodes of the charging MOSFET 5, and to the output control portion 20. As a result, high breakdown voltage components must be used for those elements, increasing the cost of the overall device.
At a time when the capacitor is not yet charged to a voltage sufficient to drive the output control portion, such as when charging is in an initial stage or when the on-time of the output MOSFET 2 is high resulting in an unsatisfactory charging capability, the output switch portion may be erroneously operated due to load potential operation. When the power source of the battery portion 30 is insufficiently charged, the depletion MOSFET 35 is at a high impedance. Under this condition, if the load potential for the output MOSFET 2 varies, current flows through a path between the drain and gate of the output MOSFET 2 created by the drain-gate capacitance. Accordingly, the gate potential of the output MOSFET 2 increases, causing a risk that terminals Ph and Pl may be short-circuited. In particular, when the semiconductor devices are incorporated into the upper and lower arms of a bridge circuit, for example, turn-on due to potential variation may cause the upper and lower arms to simultaneously turn on, thereby causing a short-circuit. Therefore, in the interest of safety, it is desired that a highly reliable switching characteristic be obtained by eliminating turn-on due to potential variation.
It is noted that in the conventional semiconductor device, the charging portion is allowed to operate only when a satisfactory potential difference appears between the load terminals Ph and Pl. Accordingly, when the on-state time of the output switch portion is long, current leakage from the diodes, MOSFETs, and other components in the output control portion will cause the voltage across the battery portion capacitor to decrease. In addition, the charging of the capacitor tends to terminate when the charging MOSFET is turned on again. Hence, the gate of the output MOSFET 2 is insufficiently charged, increasing the loss in the output MOSFET 2. Consequently, it is difficult to obtain the switching characteristic specified by the control signal.